Net Investment Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment Income Net [Abstract]
Interest income	$ 443,100	$ 499,500	$ 420,800
Dividend income	28,000	37,900	78,500
Investment expenses	(28,600)	(28,900)	(31,500)
Partnerships and other investment results	48,400	41,700	32,700
Net investment income	$ 490,856	$ 550,241	$ 500,534